The disclosure of Japanese Patent Application No. HEI 10-364670 filed on Dec. 22, 1998 including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of Invention
The present invention relates to an emission control apparatus and method for purifying exhaust gas emitted from an internal combustion engine.
2. Description of Related Art
Many internal combustion engines of, for example, motor vehicles, have in the exhaust system thereof a catalyst device in which precious metals, such as platinum, palladium and the like, are supported as catalysts in order to eliminate or lessen harmful exhaust gas components, for example carbon monoxide (CO), oxides of nitrogen (NOx), hydrocarbons (HC) and the like, before letting out the components into the atmosphere.
A typical catalyst device causes HC and CO present in exhaust gas to react with O2 present in exhaust gas thereby oxidizing HC and CO into H2O and CO2, and causes NOx present in exhaust gas to react with HC and CO present in exhaust gas thereby reducing NOx into H2O, CO2 and N2.
At the time of start of an internal combustion engine, relatively large amounts of unburned gas components, such as unburned hydrocarbons (HC) and the like, are emitted because unstable combustion is caused by relatively low engine temperature and, at the same time, the engine air-fuel ratio is set lower than the theoretical air-fuel ratio (shifted to the fuel-rich side) in order to facilitate the starting of the engine.
The catalyst device of an internal combustion engine becomes able to significantly lessen the harmful exhaust gas components when the temperature of the device reaches or exceeds a predetermined activation temperature. Therefore, when the activation temperature has not been reached, for example, at the time of cold start of the engine, the catalyst device cannot significantly lessen unburned gas components, which are emitted in large amounts in such a situation.
A known measure against the aforementioned problem is an engine exhaust gas removing apparatus described in Japanese Patent Application Laid-Open No. HEI 4-194309. The engine exhaust gas removing apparatus includes a catalytic converter disposed in an exhaust passage, a bypass passage connected to the exhaust passage and bypassing the catalytic converter, a filter chamber disposed in the bypass passage, a recovery passage connected between a portion of the bypass passage extending downstream of the filter chamber and a portion of the exhaust passage extending upstream of the catalytic converter. In addition, a first open-close valve opens and closes the exhaust passage portion upstream of the catalytic converter, a second open-close valve opens and closes the recovery passage, a third open-close valve opens and closes a portion of the bypass passage extending downstream of the connecting portion between the bypass passage and the recovery passage, and a flow adjusting valve adjusts the amount of exhaust gas flowing into the filter chamber provided in the bypass passage.
The filter chamber adsorbs unburned exhaust gas components when a predetermined temperature has not been reached, and the filter chamber desorbs the adsorbed unburned gas components when the predetermined temperature has been reached or exceeded. When the catalytic converter is not activated, the engine exhaust gas removing apparatus constructed as described above completely closes the first and second open-close valves and fully opens the third open-close valve and the flow adjusting valve in order to prevent exhaust gas from flowing into the catalytic converter. Therefore, the entire amount of exhaust gas is led to the exhaust passage portion downstream of the catalytic converter, via the bypass passage, so that unburned gas components in the exhaust gas are collected in the filter chamber.
When the catalytic converter is activated, the engine exhaust gas removing apparatus fully opens the first and second open-close valves and completely closes the third open-close valve and adjusts the flow adjusting valve to a desired opening, so that a major portion of exhaust gas flows into the catalytic converter and a small portion of exhaust gas flows into the filter chamber, and so that exhaust gas let out of (desorbed from) the filter chamber is led to the exhaust passage portion upstream of the catalytic converter via the recovery passage. In this situation, unburned gas components desorbed from the filter chamber are led to the exhaust passage portion upstream of the catalytic converter via the recovery passage, so that the unburned gas components, together with exhaust gas flowing from an upstream portion of the exhaust passage, flows into the catalytic converter and is subjected to the converting processes in the catalytic converter.
The catalytic converters that are disposed in the exhaust systems of internal combustion engines are represented by three-way catalyst devices, NOx-lessening catalyst devices and the like. The catalytic converters represented by these devices are able to eliminate or lessen unburned gas components and harmful gas components present in exhaust gas provided that the air-fuel ratio of inflowing exhaust gas is within a predetermined range. Therefore, when unburned gas components desorbed from the filter chamber are to be converted by a catalytic converter as mentioned above, it is necessary to set the air-fuel ratio of exhaust gas containing the unburned gas components to a predetermined air-fuel ratio.
In a technology proposed in conjunction with the aforementioned need in internal combustion engines, generally termed air-fuel ratio feedback control is performed in which the air-fuel ratio of exhaust gas flowing into the catalytic converter is detected, and the amount of fuel injected is adjusted so as to bring the actual air-fuel ratio of exhaust gas flowing into the catalytic converter to a desired air-fuel ratio.
It is also known that the catalytic converter has a certain oxygen storing capacity (OSC) and therefore is able to significantly lessen harmful gas components by utilizing the oxygen storing capacity even if the exhaust air-fuel ratio temporarily changes.
The engine exhaust gas removing apparatus described in Japanese Patent Application Laid-Open No. HEI 4-194309 opens the recovery passage so that the filter chamber releases the unburned gas components at the same time when the catalytic converter is activated. In some cases, therefore, the air-fuel ratio of exhaust gas flowing into the catalytic converter becomes an excessively rich ratio, so that harmful gas components and unburned gas components in exhaust gas are not processed by the catalytic converter. Thus, there is a problem of deterioration of emissions.
It may be conceivable to achieve a lean air-fuel ratio of exhaust gas in accordance with the amount of desorbed unburned gas components in the aforementioned case by reducing the fuel injection amount through utilization of the oxygen storing capacity of the catalytic converter and execution of the air-fuel ratio feedback control, so that the exhaust air-fuel ratio resulting from addition of the desorbed unburned gas components to exhaust gas becomes a predetermined air-fuel ratio. However, if large amounts of unburned gas components become desorbed from the filter chamber in unison, the amount of oxygen pre-stored in the catalytic converter due to the oxygen storing capacity is instantly consumed, so that it may become impossible to process the unburned gas components in the catalytic converter before the air-fuel ratio feedback control is reflected in the actual exhaust air-fuel ratio. Furthermore, the engine air-fuel ratio temporarily becomes an excessively lean air-fuel ratio, so that the operating condition of the internal combustion engine may become unstable.
Still further, in order to realize the air-fuel ratio feedback control, it is necessary to provide an air-fuel ratio sensor or the like in a portion of the exhaust passage extending upstream of the catalytic converter. However, ordinary air-fuel ratio sensors are able to detect accurate air-fuel ratio provided that the exhaust air-fuel ratio is within a predetermined range. If the exhaust air-fuel ratio becomes a rich air-fuel ratio beyond the detection range of such an ordinary air-fuel ratio sensor due to large amounts of unburned gas components desorbed from the filter chamber, the air-fuel ratio sensor fails to detect an accurate exhaust air-fuel ratio, so that the precision of the air-fuel ratio feedback control deteriorates, resulting in deterioration of emissions.
Accordingly, it is an object of the invention to prevent deterioration of emissions from being caused by unburned gas components desorbed from an adsorbent provided in an internal combustion engine system for adsorbing unburned gas components of exhaust gas, by preventing the unburned gas components desorbed from the adsorbent from flowing into an exhaust gas-purifying catalyst device in unison.
In accordance with an aspect of the invention, an emission control apparatus includes a catalyst device provided in a main exhaust passage of the internal combustion engine for lessening a harmful gas component of exhaust gas, a bypass passage bypassing a portion of the main exhaust passage that is located upstream of the catalyst device, an adsorbent that is provided in the bypass passage and that adsorbs an unburned gas component of exhaust gas when having a temperature below a predetermined temperature, and that releases the unburned gas component when having a temperature equal to or higher than the predetermined temperature. In addition, a passage switching device switches between flow of exhaust gas to the main exhaust passage and flow of exhaust gas to the bypass passage, and flow that occurs through the bypass passage when the unburned gas component is desorbed from the adsorbent is controlled so that the flow through the bypass passage assumes a proportion equal to or less than a predetermined proportion relative to a flow through the main exhaust passage.
In the emission control apparatus constructed as described above, the passage switching device operates so as to cause the entire amount of exhaust gas to flow through the bypass passage when the exhaust gas-purifying catalyst is not activated, for example, at the time of cold start of the internal combustion engine. In this situation, the entire amount of exhaust gas discharged from the internal combustion engine passes through the adsorbent disposed in the bypass passage. Therefore, unburned gas components present in exhaust gas are entirely adsorbed to the adsorbent and are not let out into the atmosphere.
After the exhaust gas-purifying catalyst is activated, the passage switching device operates so as to cause exhaust gas to flow through both the main exhaust passage and the bypass passage. In this situation, exhaust gas from the internal combustion engine flows into the exhaust gas-purifying catalyst device via the main exhaust passage and the bypass passage. However, the amount of the flow through the bypass passage is controlled so that the flow through the bypass passage assumes a proportion equal to or less than (i.e., not greater than) a predetermined proportion relative to the flow through the main exhaust passage. Therefore, a major portion of exhaust gas flows into the exhaust gas-purifying catalyst device via the main exhaust passage, and the small remainder portion flows into the exhaust gas-purifying catalyst device via the bypass passage.
That is, the flow of exhaust gas through the adsorbent becomes small, so that the temperature increasing rate of the adsorbent becomes low. As a result, the desorption of unburned gas components from the adsorbent proceeds at a slow rate, and an undesired event in which the unburned gas components adsorbed to the adsorbent are desorbed therefrom and flow into the exhaust gas-purifying catalyst device in unison (i.e., in large amounts at one time) is prevented. Therefore, the air-fuel ratio of exhaust gas flowing into the exhaust gas-purifying catalyst device will not deviate from a range of air-fuel ratio in which the exhaust gas-purifying catalyst device is able to significantly lessen harmful exhaust gas components.
Furthermore, since the flow through the bypass passage is controlled so that the flow through the bypass passage assumes a substantially constant proportion equal to or less than the predetermined proportion relative to the flow through the main exhaust passage, the proportion of the amount of exhaust gas flowing into the exhaust gas-purifying catalyst device via the bypass passage to the amount of exhaust gas flowing into the exhaust gas-purifying catalyst device via the main exhaust passage becomes substantially constant. Therefore, even when the flow of exhaust gas discharged from the internal combustion engine changes, the change in the air-fuel ratio of exhaust gas flowing into the exhaust gas-purifying catalyst device is curbed.
The flow through the bypass passage also may be controlled so that the flow through the bypass passage becomes a constant flow regardless of the flow through the main exhaust passage.
The flow through the bypass passage may be controlled by arranging the bypass passage at such a position that a differential pressure occurring between an exhaust gas inlet portion of the bypass passage and an exhaust gas outlet portion of the bypass passage becomes equal to or less than a predetermined pressure. The flow through the bypass passage varies in accordance with the pressure difference between the exhaust gas inlet portion and the exhaust gas outlet portion. Therefore, by keeping the pressure difference between the exhaust gas inlet portion and the exhaust gas outlet portion equal to or less than the predetermined pressure, the flow through the bypass passage can be kept equal to or less than the predetermined flow.
The bypass passage may be disposed in such a manner that the exhaust gas inlet portion of the bypass passage is disposed at a position in the main exhaust passage adjacent to and upstream of the passage switching device, and such that the exhaust gas outlet portion of the bypass passage is disposed at a position in the main exhaust passage adjacent to and downstream of the passage switching device.
If the exhaust gas inlet portion and the exhaust gas outlet portion of the bypass passage are disposed at adjacent positions as described above, the pressure difference between the exhaust gas inlet portion and the exhaust gas outlet portion and the exhaust pulsation phase difference therebetween become small, so that the flow through the bypass passage can be made very small.
The adsorbent and the bypass passage may be arranged coaxially with the main exhaust passage. In this case, it is also possible to arrange an annular adsorbent and an annular bypass passage around the main exhaust passage in order to miniaturize the emission control apparatus and thereby make it easier to install the device in a vehicle. Furthermore, the adsorbent and the bypass passage may be arranged coaxially with the catalyst device disposed in the main exhaust passage. As a result, it becomes possible to shift the position of the exhaust gas-purifying catalyst device in the exhaust system closer to the internal combustion engine. In such an arrangement, higher-temperature exhaust gas flows through the exhaust gas-purifying catalyst device, so that the catalyst device can be activated in a shorter period (i.e., at an earlier time).
If the adsorbent, the bypass passage and the main exhaust passage are coaxially arranged, the bypass passage may be provided with at least one retainer for preventing the bypass passage from deforming. In this case, the bypass flow control device adjusts the flow through the bypass passage by adjusting one of a shape, a number and a position of the at least one retainer.
Since the provision of retainers in the bypass passage reduces the passage area of the bypass passage, a desired exhaust gas flow through the bypass passage can be achieved by optimizing the shape, the number and/or the position of the retainers.
Furthermore, a damper chamber for damping exhaust pulsation may be formed in the pathway of the bypass passage.
Each of the exhaust gas inlet portion of the bypass passage and the exhaust gas outlet portion of the bypass passage may have a shape such that a differential pressure occurring between the exhaust gas inlet portion and the exhaust gas outlet portion becomes equal to or less than a predetermined pressure.
The emission control apparatus may further include an air-fuel ratio sensor that detects at least an air-fuel ratio of exhaust gas downstream of the adsorbent, and an air-fuel ratio adjusting device for adjusting the air-fuel ratio of exhaust gas so that a value of an output signal of the air-fuel ratio sensor becomes equal to a target air-fuel ratio. Since the flow through the bypass passage is controlled such that the flow of exhaust gas through the adsorbent assumes a proportion equal to or less than the predetermined proportion relative to the flow of exhaust gas through the main exhaust passage, the change in the air-fuel ratio of exhaust gas flowing into the exhaust gas-purifying catalyst device is curbed. Therefore, the air-fuel ratio adjusting device can easily bring the air-fuel ratio of exhaust gas flowing into the exhaust gas-purifying catalyst device to a desired air-fuel ratio, that is, an air-fuel ratio that optimizes the exhaust gas-purifying efficiency of the catalyst device.
The flow through the bypass passage may be controlled by detecting a pressure difference between the exhaust gas inlet portion of the bypass passage and the exhaust gas outlet portion of the bypass passage, and then controlling the passage switching device so that the pressure difference becomes equal to or less than a predetermined value. Furthermore, a detector may detect at least one of an amount of the unburned gas component present in exhaust gas and an air-fuel ratio of exhaust gas downstream of the adsorbent, and may control the passage switching device so that the at least one of the amount of the unburned gas component and the air-fuel ratio of exhaust gas becomes constant.